The present invention relates to image forming devices such as copying machines, facsimile devices, printers and the like utilizing an electrophotograph method.
In the prior art, as is disclosed in Japanese Laid-Open Patent Publication No. 60-213977, a fixing device is proposed for sensing the surface temperature of the heating roll and controlling the temperature of the heater installed in the heating roll based on the sensed results, wherein the heater is heated to a target temperature calculated based on the surface temperature of the heating roll at the starting time of the control and the heat-up rate of the heating roll, and then stopping the heater for a predetermined time in order to prevent overshoot of the target temperature.
The above-mentioned device is suitable in a low-speed fixing device designed to have a short warm-up time. However, in a high-speed fixing device with a larger heating roll diameter and a larger pressure roll diameter, the consequence from the overshooting of the heater is rather small, and in contrast, with the fixing heat capacity being insufficient at initial printing times, the problem of the fixing ability of initial prints being poor just after the power was turned on was more serious than the overshooting of the heater. A fixing device solving such problem is disclosed in Japanese Laid Open Patent Publication No. 4-58274.
In the above-mentioned fixing device, the heat necessary for fixing the toner provided from the heating roll with high heat conduction rate to the sheet is absorbed in the pressure roll at the initial printing time just after the power is turned on, and the surface temperature of the heating roll decreases.
Further, since the pressure roll has a low temperature, the back surface of the printing sheet is cooled, resulting in the decrease of the fixing efficiency.
Therefore, in the fixing device disclosed in Japanese Laid-Open Patent Publication No. 4-58274, the power to the heater was controlled so that it is heated higher than the predetermined fixing temperature for a certain period of time after the signal to turn on the power to the heater in the heating roll is received.